2101 is the model number, so it's just a jcm 900
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yeah, 2101 is the model number

there was a whole bunch of different JCM900s (at least in the format they came, 100 watt, 50 watt, head, 1x12" combo, 2x12" combo....)
2101 is the 100w 1x12" combo (as evident out of the rest of the ad)